The present invention relates generally to the correction of horizontal deflection linearity for picture tubes or cathode ray tubes, and more particularly to an apparatus for processing the waveform of a sawtooth current fed to a horizontal deflection coil in order to improve the linearity of horizontal deflection on a screen of a picture tube.
An important problem in a picture tube provided with a screen having a greater radius of curvature than that of the locus of electron beams relates to the correction of the linearity of the horizontal deflection. The distance from the deflection center to the screen becomes greater at the screen edges than at the center of the screen and therefore the per-unit-time travelling distance of the electron beam on the screen increases progressively from the center of the screen toward the edges thereof, resulting in the occurrence of picture distortion at the edge portions thereof.
In order to avoid this problem it is known that a capacitor, blocking dc components, is connected in series to a horizontal deflection coil to which a ramp current is fed from a horizontal deflection output circuit so that the waveform of the ramp current is processed to have a S-configuration with resonance action caused by the capacitor and the horizontal deflection coil. However, one of problems with such an arrangement is that it is impossible to effect the correction with respect to the fact that the left edge portions of the screen is different from the right edge portion thereof in linearity because of the variations in the characteristic of the horizontal deflection coil and that different distortions occur at the right and left edge portions thereof due to the application of an exponential component to the ramp current. Furthermore, when there is a need to operate a picture tube with a variable horizontal deflection frequency to make it compatible with different scanning systems, a switching device will be required for selecting one from a plurality of capacitors in accordance with the change of the horizontal defletion frequency. However, with such a prior art technique, it is required to perform the switching operation with a power supply being cut off in order to prevent an electric arc from occuring at the contacts of the switching device, resulting in complex switching operation.